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Persistent phono hum in Altronics kit 5513
Persistent phono hum in Altronics kit 5513
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Old 8th February 2015, 12:41 AM   #1
ecodan is offline ecodan  Australia
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Default Persistent phono hum in Altronics kit 5513

Hi all, it thought I had hum problem in my turntable, however after some time I discovered the hum was still present unchanged when I unplugged everything from my phono stage of my preamp.

My phono preamp is an Altronics phono kit as per the design in the silicon chip magazine. This was mounted in a plastic kit case as part of a passive preamp, which consists of a input rotary switch and a volume knob.

Hum is only present when the phono stage is selected, I have also switched to a better metal box since the photo was taken (using the same layout) changed to a metal box for better ground shielding but with no appreciable improvement.

The kit offers a suggested upgrade to a grounded power supply, but suggests this is normally unnecessary.

Any thoughts (especially from those who have built this kit) as to why I'm having this problem?

Click the image to open in full size.
Note bank of resistors is for quick input load change
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Old 8th February 2015, 12:53 AM   #2
nigelwright7557 is offline nigelwright7557  United Kingdom
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I usually have hum problems if my zero volt line isn't earthed.

It depends on what else is earthed in the signal chain.
If the amplifier zero volt line is earthed the preamp shouldn't need it, in fact 2 earths can cause a earth loop.

If your case is metal that should be earthed too.
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Old 8th February 2015, 01:19 AM   #3
ecodan is offline ecodan  Australia
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Quote:
Originally Posted by nigelwright7557 View Post
I usually have hum problems if my zero volt line isn't earthed.

It depends on what else is earthed in the signal chain.
If the amplifier zero volt line is earthed the preamp shouldn't need it, in fact 2 earths can cause a earth loop.

If your case is metal that should be earthed too.
Thanks for the quick reply Nigel. Power amp is a Quad 303, which is plugged directly into the wall using an earthed lead, however I haven't checked if the amp uses the earth pin. The turntable did not have an earthed lead, however it made no difference when I added one. Turntable is grounded to the preamp, but as I said it hums with or without the turntable connected if the phono stage is selected.

The hum is not earth shattering, in fact not noticeable at normal listening volume unless using headphones, but rapidly becomes loud beyond that point. I know it reduces sound quality so I want to to reduce it further.

Meanwhile, I am Very happy with the sound quality improvement over my old Quad 33, which despite revision, remained Woolley!
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Old 8th February 2015, 01:25 AM   #4
nigelwright7557 is offline nigelwright7557  United Kingdom
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Routing of wires is important too.
Don't run mains wires close to signal wires.
Same goes for the mains transformer don't fit it close to signal wires or pcb.
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Old 8th February 2015, 01:56 AM   #5
ecodan is offline ecodan  Australia
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Quote:
Originally Posted by nigelwright7557 View Post
Routing of wires is important too.
Don't run mains wires close to signal wires.
Same goes for the mains transformer don't fit it close to signal wires or pcb.
Transformer is external, incoming power cables run near the power circuit next to a shield. Case is shielded and I have tried wrapping grounded foil around the cable connected to the case that is grounded to a specified part of the circuit board. Also checked no other power cords in vicinity, besides it get no hum when switched to other inputs so out put is not picking anything up.

Obviously the board is not grounded well enough somewhere
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Old 8th February 2015, 01:59 AM   #6
nigelwright7557 is offline nigelwright7557  United Kingdom
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I guess the pcb is the only thing left.
Do you still get hum with the input socket shorted out ?
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Old 8th February 2015, 03:43 AM   #7
ecodan is offline ecodan  Australia
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Quote:
Originally Posted by nigelwright7557 View Post
I guess the pcb is the only thing left.
Do you still get hum with the input socket shorted out ?
No it still hums, but I accidentally connected the outside of the rca socket to the case and it halved. Then I connected it to the centre of the phono stages own output volume control spindle (mounted off the pcb as a posed to my master volume on the front of the case) ???
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Old 8th February 2015, 03:49 AM   #8
ecodan is offline ecodan  Australia
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Default Solution.

Ahh, footnote mentions can ground banana plug to case if mains hum is a problem. Shorted it with some wire and seems to 100% cure problem. Will implement properly and see.
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Old 8th February 2015, 05:48 AM   #9
ecodan is offline ecodan  Australia
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Made a huge difference. Wouldn't call it fixed, but it's as quiet as a graveyard (on a windy night), when u turn it up most of the way. Noise doesn't creep in to well above my maximum comfortable listening volume.

Thanks Nigel

Last edited by ecodan; 8th February 2015 at 05:51 AM.
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Old 8th February 2015, 05:53 AM   #10
ecodan is offline ecodan  Australia
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Funny side note, I live 15min away from Carlisle in Western Australia!
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